This invention relates to a fuel cell system having a component that utilizes shaft work.
Alexander Grove invented the first fuel cell in 1839. Since then most of the fuel cell development has been primarily limited to applications supported by the government such as the United States National Aeronautics and Space Administration (NASA), or in utility plants. However, recent developments in materials of construction and processing techniques have brought fuel cell development closer to significant commercial applications. A primary advantage of fuel cells is that fuel cells can convert stored energy to electricity with about 60-70 percent efficiency, with higher efficiencies theoretically possible. Further, fuel cells produce virtually no pollution. These advantages make fuel cells particularly suitable for vehicle propulsion applications and make fuel cells a potential replacement for the internal combustion engine which operates at a less than 30 percent efficiency and can produce undesirable emissions.
A fuel cell principally operates by oxidizing an element, compound or molecule (that is, chemically combining with oxygen) to release electrical and thermal energy. Thus, fuel cells operate by the simple chemical reaction between two materials such as a fuel and an oxidant. Today, there are variety of fuel cell operating designs that use many different fuel and oxidant combinations. However, the most common fuel/oxidant combination is hydrogen and oxygen (usually in the form of air).
In a typical fuel cell, hydrogen is burned by reacting the hydrogen with oxygen from air to produce water, electrical energy and heat. This is accomplished by feeding the hydrogen over a first electrode (anode), and feeding the oxygen over a second electrode (cathode). The two electrodes are separated by an electrolyte which is a material that allows charged molecules or xe2x80x9cionsxe2x80x9d to move through the electrolyte. There are several different types of electrolytes that can be utilized including the acid-type, alkaline-type, molten-carbonate-type and solid-oxide-type. The so-called PEM (proton exchange membrane) electrolyte (also known as a solid polymer electrolyte) is an acid-type, and potentially has high-power and low-voltage, and thus are desirable for vehicle applications.
Although compressed or liquefied hydrogen could be used to operate a fuel cell in a vehicle, to date this is not practical. The use of compressed or liquefied hydrogen ignores the extensive supply infrastructure currently being used to supplying gasoline for internal combustion engine automobiles and trucks. Consequently, it is more desirable to utilize a fuel source such as methanol or gasoline to provide a hydrogen source for the fuel cell. However, the methanol or gasoline must be reformed to provide a hydrogen gas source. This is accomplished by using methanol or gasoline fuel processing equipment and hydrogen cleanup or purification equipment.
Fuel cell systems often include a fuel processing section which reforms a fuel, preferably an organic based fuel such as methanol or gasoline, to produce hydrogen and a variety of other byproducts. However, these reforming processes are endothermic and require energy input to drive the reforming reaction. This energy input and the breakdown of the fuel during the reforming process increases the pressure of the reformer exhaust. As a result, after the hydrogen stream exits the reformer and when it is delivered to the fuel cell, the hydrogen stream is under pressure at about 3 bars.
Consequently, both sides of the above described fuel cell membrane must have the same pressure. Otherwise, the membrane would flex back and forth, and the catalyst on the membrane that is used to reduce hydrogen molecules would be damaged resulting in poor performance or failure of the system. Thus the oxidant or air being supplied to the fuel cell stack must be compressed to the same pressure as the hydrogen stream.
Additionally, other fuel cell processing components such as preferential oxidation reactors may require compressed air to operate. These compressed air requirements are a significant drain on the fuel cell system. Approximately 10 percent of the power generated by the fuel cell stack goes back into compressing air under most of the current fuel cell systems. Thus it would be desirable to develop a system which reduced the amount of electricity utilized to run air compressors in a fuel cell system.
The present invention provides alternatives to and advantages over the prior art.
The invention includes a fuel cell system and process using a Rankine cycle to produce shaft work to operate a fuel cell system component. The shaft work may be used to drive an air compressor to deliver compressed air to a fuel cell system component. The steps of the Rankine cycle include pumping a liquid working fluid to an elevated pressure, heating the fluid to a gaseous state, expanding the high temperature and high-pressure gas through expander to produce shaft work used to drive a fuel cell system component such as an air compressor, and then removing energy from the cooling fluid gas to change the gas back to a liquid, and repeating the cycle. The liquid fluid can be heated by an external boiler, or one of the components of the fuel cell system such as a combustor or the fuel cell stack.
In a preferred embodiment, the invention includes a fuel cell system and process that utilizes a fuel cell to co-generate electricity and shaft work. The system utilizes waste thermal energy, generated by a fuel cell stack, to produce shaft work that can be used to drive an air compressor. The air compressor compresses process air needed by the fuel cell stack or other fuel cell system components. To accomplish these tasks a Rankine cycle is used in the cooling system of the fuel cell stack to recover the waste thermal energy.
In comparison to traditional methods of cooling the fuel cell stack, the present invention allows for a reduction in the ancillary power equipment, a decrease in the size of the cooling system, and a method of the heating the stack in cold start conditions. The system converts the low-grade thermal energy produced by the fuel cell stack to shaft work, and then uses the shaft work to compress and move process air needed by the fuel cell stack. As a result, the electrical motor used to compress process air in conventional systems is displaced, and the size of the stack can be reduced because the electricity requirement of the conventional air compressor it is eliminated. The size of the cooling system is reduced by the amount of energy converted to shaft work. The net result of these two features is a reduction in the size of cooling system radiators, a concern with conventional cooling systems.
Another benefit of the present invention is that the system may be used heat the fuel cell stack in cold start conditions. The system directs the coolant through a supplemental boiler/super heater when the system is below a predetermined temperature. The supplemental boiler/super heater heats the cooling fluid which in turn heats the fuel cell stack. This greatly reduces the startup time of the system.
These and other objects, features and advantages of present invention will become apparent from the following brief description of drawings, detailed description of preferred embodiments, and appended claims and drawings.